Stancie and I were cruising along in our Baron in nice smooth air with the autopilot doing the flying when I noticed the control wheel start to twitch. The movements were small, and not all that unusual because autopilots often are not happy with perfectly smooth air. They want to be doing something, so often there are tiny control movements from even a perfectly functioning autopilot when the air is perfectly smooth.
But as I watched the control wheel closely I saw the left-right excursions growing in amplitude. And also in frequency. There was a layer of clouds ahead and I hoped a few little bumps would give the autopilot something to think about and it would settle down.
About 10 minutes after I noticed the first twitches by the control wheel the autopilot swung the wheel left, then right very quickly using about half of the available control travel. And then the autopilot servo twisted the ailerons full left driving the control wheel quickly to full travel.
What I had experienced was an autopilot “hard over”, one of the critical failures in any autopilot system, and type of failure the certification standards take very seriously.
All reasonably recently designed and certified autopilots have a master interrupt switch under your thumb on top of the outboard horn of the control wheel, or on a stick if the airplane is so equipped. Pressing and holding that button removes electrical power from the autopilot servos, and even more importantly, from the pitch trim servo. You’re first reaction when the autopilot does something you don’t like, or don’t expect, should be to press and hold that button. And it’s usually colored red to emphasize its critical importance.
But even as I was getting my thumb on the interrupt button I rolled the wheel back to level before the airplane had a chance to enter much of a bank. Autopilot servos have some kind of a clutch system that can be over-ridden by a pilot using normal strength on the controls. The servo was trying to drive full left aileron, but I could stop the action by simply applying more force than the clutch allowed.
During a hard over servo failure in roll simply holding the wheel firmly and slipping the servo clutch has no dire consequences. The system is doing its best to bank the airplane, but any pilot has enough strength to prevent the controls from moving. But if the failure had been in the autopilot pitch control system slipping the clutch by holding the wheel and overpowering the servo would have created huge problems, and has been the cause of a number of crashes.
The danger in slipping the pitch servo clutch is that you will almost certainly cause the pitch trim servo to run in opposition to your efforts. Autopilot clutches are really pretty wimpy, but the pitch trim system, even in piston singles, is powerful and, depending on flap configuration, could even overpower the human pilot.
Autopilots keep an airplane in pitch trim the same way a human does. The autopilot has a servo that moves the elevator to hold the commanded attitude, or airspeed or to track a vertical path. And, like the human pilot, the autopilot senses pitch trim changes as a force on the controls. The elevator servo and its electronics then send a command to the pitch trim system to run the pitch trim servo until the elevator control force is neutralized.
So, if the elevator servo goes hard over as my roll servo did, and you grab the controls and stop the elevator motion, the autopilot will think it needs to run the pitch trim to relieve the force. As you hold the elevator against the servo and slip the clutch, the pitch trim servo will run in opposition. What you will experience is an increasing control force against what you’re holding, not because the elevator servo is gaining strength, but because the pitch trim is moving opposite your efforts. Before long you could have more stick force than you can hold and you may not understand the reason. Even if you eventually get your thumb on the interrupt button the control force will still be there because it’s trim force, not autopilot servo effort, that you are feeling.
Bottom line is that it’s perfectly okay to grab the controls when the autopilot does something strange and stop the control movement, but you must also immediately get your thumb on the interrupt button and remove power from the system. And be sure to read the manuals and understand your autopilot. In many systems the red button interrupts power only as long as you hold it down. In that style system you need to turn the autopilot off using another switch—or the circuit breaker—while continuing to hold the red button down.
Autopilots are essential for single pilot IFR as far as I’m concerned—and the FAA, too, if the trip is for hire—but you need to keep an eye on them and understand how to handle things when George has a bad day.